The present invention relates to polymer compositions having low scorch during extrusion. Furthermore, it relates to articles, in particular multilayered articles like power cables, comprising such polymer compositions.
In general, the degree of unsaturation of polyolefins is dependent on specific conditions chosen for the polymerisation process. This is true for high pressure as well as low pressure conditions. If e.g. polyethylene is produced by radical polymerisation (so-called low-density polyethylene LDPE), the number of double bonds within the polymer is usually quite low. However, in many situations, it is desirable to use polymers having a higher degree of unsaturation which may serve to introduce functional groups into the polymer molecule or to effect crosslinking of the polymer.
The crosslinking of polyolefins like polyethylene is relevant for many applications, such as extrusion (e.g. of tubes, cable insulating material or cable sheathing), blow moulding, or rotational moulding. In particular in cable technology, crosslinking is of special interest since deformation resistance at elevated temperature of the cable can be improved.
In WO 93/08222, an unsaturated low-density polyethylene (LDPE) having improved crosslinking properties was prepared by high pressure radical polymerisation of ethylene and a specific type of polyunsaturated comonomers. The increased amount of unsaturation of the LDPE copolymer increases the crosslinking activity when combined with a crosslinking agent.
As indicated above, crosslinkable polyolefins are of interest for applying coating layers on power cables by extrusion. In such an extrusion process of a power cable, the metallic conductor is generally first coated with a semiconductive layer, followed by an insulating layer and another semiconductive layer. These layers are normally crosslinked and are normally made of cross-linked ethylene homopolymers and/or ethylene copolymers.
Cross-linking can be effected by adding free-radical forming agents like peroxides to the polymeric material prior to or during extrusion. The free-radical forming agent should preferably remain stable during extrusion performed at a temperature low enough to minimize the early decomposition of the peroxide but high enough to obtain proper melting and homogenisation. Furthermore, the crosslinking agent should decompose in a subsequent cross-linking step at elevated temperature. If e.g. a significant amount of peroxide already decomposes in the extruder, thereby initiating premature crosslinking, this will result in the formation of so-called “scorch”, i.e. inhomogeneity, surface uneveness and possibly discolouration in the different layers of the resultant cable. Thus, any significant decomposition of free-radical forming agents during extrusion should be avoided. On the other hand, thermal treatment at the elevated temperature of the extruded polyolefin layer should result in high crosslinking speed and high crosslinking efficiency.
In EP-A-0453204 and EP-A-0475561, 2,4-diphenyl-4-methyl-1-pentene is added to polymer compositions to suppress the formation of scorch. These applications do not relate to unsaturated polyolefins.
Furthermore, during the crosslinking step, by-products can be generated due to decomposition of crosslinking agents. Most by-products are captured within the cable and the volatile fraction thereof has to be removed in a so-called degassing step. The more by-products generated, the longer the degassing time and/or the higher the degassing temperature. However, mild degassing conditions would be preferred. Milder degassing conditions would also reduce the risk of damaging the cables during the degassing step.